1. Field of the Invention
The present invention relates in general to a drill, and more particularly to such a drill that is to be used to enlarge a hole previously formed in a workpiece, for example, in a die-casting process.
2. Discussion of the Related Art
In general, most of commercially available drills have a point angle of 120-140°. However, such a drill having the point angle of 120-140° has a difficulty in an operation for drilling a hole in a work surface which is not perpendicular to an axis of the drill, or which is curved or otherwise shaped to be brought into contact with a radial corner of the drill earlier than with a point of the drill, upon biting of the drill onto the work surface. This is because the drill is in contact at only one of its diametrically opposite portions with the work surface in an initial stage of the drilling operation, thereby increasing a radial component of a cutting resistance or force, where this radial component forces the point of the drill outwardly in the radial direction. Due to the increased radial component of the cutting force, the point of the drill is deviated from a target position, thereby resulting in a low positional accuracy of the drilled hole.
In the interest of avoiding such a reduction in the positional accuracy of the drilled hole, there is proposed a drill as disclosed in JP-A-2004-141970 (publication of unexamined Japanese Patent Application laid open in 2004), which is given a point angle as large as 170-180°. Such a relatively large point angle is somewhat effective to reduce the radial component of the cutting force acting on the drill in an initial stage of the drilling operation, thereby restraining the deviation of the point of the drill from the target position.
In the drill disclosed in JP-A-2004-141970, each of cutting edges (provided in an axially distal end portion of its cylindrical main body) has a curved portion which is curved to be concaved rearwardly as viewed in a rotating direction of the drill, so that chips produced as a result of cutting of a workpiece are compressed in a radial direction of the drill, whereby the chips can be easily broken. Owing to the easy breakage of the chips, it is possible to prevent the chips from being packed between the drill and the workpiece and also prevent the chips from being rolled around the drill. However, when this drill is used to enlarge a hole previously formed in a workpiece, for example, in a die-casting process, the drill is likely to be forced outwardly in the radial direction by the radial component of the cutting force. That is, in this instance, the curved portion of each cutting edge brought into contact with the workpiece causes the radial component of the cutting force to act on the drill outwardly in the radial direction rather than inwardly in the radial direction, thereby deviating the point of the drill from a target position and accordingly resulting in low positional accuracy of the machined hole.